Abstract

This study focused on the effectiveness of implementing a neuromuscular and proprioceptive sports (soccer) specific training program to reduce the incidence of ACL ligament injuries in 14-18 year old female soccer players. The PEP (Prevent Injury, Enhance Performance) ACL Prevention Program focused on biomechanical risk factors, stressing avoidance of high-risk behaviors and increasing kinesthetic awareness of the enrolled subjects. A comprehensive educational videotape and literature packet was distributed to the coach of each enrolled soccer team in the Fall 2000 season. The video consisted of a twenty-minute alternative warm-up program focusing on injury awareness and avoidance techniques, lower extremity strength and trunk stability, flexibility, progressive plyometrics, and sports specific agilities. The program was completed two to three times a week over the duration of the 2000 and 2001 soccer seasons (twelve weeks). The video emphasized proper biomechanical control via visual and auditory cues, including correct and incorrect performance of landing techniques, plyometrics, agilities and strengthening exercises. Random site visits were made to address quality assurance and compliance issues. During the 2000 season, 1041 female subjects (52 teams) and 1902 players (95 teams) served as the age and skill matched non-randomized control. Of the enrolled subjects, 2 confirmed ACL tears (.2 ACL injuries/athlete/1,000 exposures) were reported versus 32 confirmed ACL tears (1.7 ACL injuries/athlete/1,000 exposures) reported from the control group. In 2000, there was an overall 88% decrease in ACL ligament injury in the enrolled subjects compared to the control group. In year two of this study (2001 season), 844 female athletes (45 teams) were enrolled in the study. There were 1913 female athletes (112 teams) served as the control. Four ACL tears were reported in the intervention group (.47 injuries/athlete/1,000 exposures) versus thirty-five ACL tears reported in the control group (1.8 injuries/athlete/1,000 exposures) (Table 1). This corresponds to an overall 74% reduction in ACL tears in the intervention group compared to an age and skill-matched control group. Utilizing a neuromuscular training program may have a direct benefit in decreasing the number of ACL ligament injuries in female soccer players.

Introduction

Anterior cruciate ligament disruption has been increasingly problematic in the lives of both recreational and competitive athletes, both physically and psychologically. These injuries can lead to an inability to perform athletically, in addition to initiating degenerative changes at the joint level. Compete ACL rupture has been very closely associated with knee instability and secondary disruption of the menisci, chondral surfaces, and the onset of osteoarthritis. In 1972, landmark legislation known as Title IX of the Educational Amendments was passed in order to enable women to participate in academic and athletic events without gender discrimination. Since this time, the advancement of women’s sports has been incredible. More than 2.36 million girls participate in high school sports, compared to 300,000 in 1972. In addition to the increase of women’s participation in sports, the incidence of severe ACL ligament injuries has also increased. Competitive team sports including soccer, basketball, and volleyball are known to require lower extremity dynamic stability to withstand the demands of cutting, decelerating, and jumping. Several prevalence studies have indicated that the number of female athletes incurring a serious ACL ligament injury exceeds that of her male counterpart by two to eight times, which would indicate a level of gender specificity.1, 12, 21, 24

The purpose of this study was to determine the effectiveness of a neuromuscular and proprioceptive performance program in decreasing the incidence of ACL injury within a select population of competitive female youth soccer players. Proprioception has been described as the acquisition of stimuli by peripheral receptors in addition to the conversion of mechanical stimuli to a neural signal that is transmitted along afferent pathways of the sensorimotor system.20 Proprioception does not include CNS processing of the incoming afferent signal, nor does it include control of efferent (outgoing) motor signals. However, this “proprioceptive” information is crucial for optimal motor performance. It is delivered to every motor control center and is utilized to garner information regarding joint position and kinesthesia (joint motion) in order to elicit active and reflexive movement. Neuromuscular control is defined as the unconscious efferent response to an afferent signal regarding dynamic joint stability. The afferent proprioceptive signals that elicit motor control can be distinguished by their role: feedback or feedforward Feedback mechanisms are a result of afferent input (force to the joint) and are reflexive in nature. The time to elicit such a reaction is longer, thus it is thought to be more heavily involved with maintaining posture and slow movement. Feedforward mechanisms are a result of preactivated preparatory activation of muscle.9

Materials & Methods

In year one of the study (2000 season), a total of 1041 female subjects (from 52 teams) between the ages of 14 and 18 participated in the first year of a three-year study. The control group consisted 1902 female soccer players (from 95 teams) participating in the same league between (age and skill matched). In year two of the study (2001 season), 844 female athletes (from 45 teams) were enrolled in the study. There were 1913 female athletes (from 112 teams) served as the non-randomized, age and skill-matched control.

The coaches of all teams within the Coast Soccer League of Southern California were contacted via central website posting and email solicitation and queried about interest in a new training program. Fifty-two (2000) and forty-five (2001) of the teams, respectively, that expressed interest were invited to participate in the program. The teams consisted of female athletes participating in a club soccer league between the ages of 14 to 18. Internal Review Board approval was granted by the Human Subjects Committee from St. John’s Hospital, Santa Monica, California.

Each team was mailed an educational videotape depicting the twenty-minute warm-up program and a supplemental literature packet. The video was designed to replace the traditional warm-up being performed by the athletes previously. The video consisted of stretching, strengthening, plyometrics, and sports specific agility drills (Table 2). Throughout the video, there is a heavy emphasis on proper technique: landing technique, stressing “soft landing”, hip and knee flexion as opposed to landing with a “flat foot” in lower extremity extension. Visual examples of proper and improper biomechanical performance were given for each individual exercise. In order to address quality assurance, a compliance form was administered to each team participating in the program. The form consisted of a checklist listing each individual component of the program. The coach was asked to observe the team completing the performance of the PEP Program in the last week of the season. The forms were then returned via mail or email to the project coordinator for assessment.

Table 2Exercises Demonstrated on Videotape

EXERCISE

DISTANCE

REPETITIONS

I. Warm-Up

Jog Line to Line

50 Yards

1

Shuttle Run

50 Yards

1

Backward Running

50 Yards

1

II. Stretching

Calf Stretch

NA

2 x 30 Seconds

Quadricep Stretch

NA

2 x 30 Seconds

Hamstring Stretch

NA

2 x 30 Seconds

Inner Thigh Stretch

NA

2 x 30 Seconds

Hip Flexor Stretch

NA

2 x 30 Seconds

III. Strengthening

Walking Lunges

20 Yards

1 Pass

Russian Hamstring

NA

30

Single Toe Raises

NA

30 Reps Bilaterally

IV. Plyometrics

Lateral Hops

6" Cone

20

Forward Hops

6" Cone

20

Single Leg Hops

6" Cone

20

Vertical Jumps

NA

20

Scissors Jumps

NA

20

V. Agilities

Shuttle Run

40 Yards

1

Diagonal Run

40 Yards

1

Bounding Run

45-50 Yards

1

The clinical criteria for confirming an ACL tear included history, physical examination by a physician, and confirmation via MRI and/or Arthroscopic procedure.

Statistical analysis included a one-way chi square analysis to test the effect of training on two distinct groups, those who received the intervention and those who did not (control).

Results

The total number of athlete exposures throughout the 2000 soccer season was 37,476 for the trained group and 68,580 for the untrained group. A total of two ACL tears confirmed by MRI were reported for the intervention group for an incidence rate of .2 ACL injuries/athlete per 1000 exposures. Thirty-two (32) ACL tears were reported for the control group to result in an incidence rate of 1.7 ACL injuries/athlete per 1000 exposures (Table 3). These results indicate an 88% overall reduction of ACL injury per individual athlete compared to a skill and age matched control athlete. When these results are analyzed using the team as a unit of analysis, the injury incidence for the intervention group is 0.04 compared to the control group, 0.37. The resultant rate ratio is equivalent to 0.11, which is statistically significant at p=0.05 with a 95% confidence interval (range 0.03 – 0.46).

Table 3Year 1 (2000)

# of Subjects

# ACL Tears

Resultant Rate Per Player

# of Teams

Resultant Rate Per Team

Trained

1041

2

.2

52

.04

Untrained

1902

32

1.7

95

.37

Total

2943

34

147

In year two (2001) of the study, the total number of athlete exposures was 30,384 for the trained group was and 68,868 for the untrained group. Four ACL tears were reported in the intervention group, with an incidence rate of .47 injuries/athlete per 1,000 exposures. Thirty-five ACL tears were reported in the control group, with an incidence rate of 1.8 injuries/athlete/1,000 exposures. This corresponds to an overall 74% reduction in ACL tears in the intervention group compared to an age and skill-matched control group in year 2 (Table 4). When these results are analyzed using the team as the unit of analysis, the injury incidence rate for the intervention group is 0.08 compared toe the control group, 0.31. The resultant rate ration is equivalent to 0.26, which is statistically signification at p=0.05 with a 95% confidence interval (range 0.03 – 0.46).

Table 4Year 2 (2001)

# of Subjects

# ACL Tears

Resultant Rate Per Player

# of Teams

Resultant Rate Per Team

Trained

844

4

.47

45

.08

Untrained

1913

35

1.8

112

.31

Total

2757

39

157

Discussion

When discussing the nature of a prophylactive injury prevention program, the training that is implemented into the curriculum is seeking to primarily address the feedforward mechanism – to anticipate external forces or loads in order to stabilize the joint, thus protecting the inherent structures.

Several intrinsic and extrinsic risk factors have been examined in order to determine the etiology of ACL tears. These risk factors include anatomic, hormonal, environmental, and biomechanical.10 The variation in morphology between males and females has been well examined, including variances in pelvic size and shape, intercondylar notch width, size of ACL, and Q angle have been examined. Throughout the menstrual cycle, fluctuations in progesterone, estrogen and relaxin hormones have been studied to determine their effect on the integrity of the ACL. Estrogen, progesterone, and relaxin receptor sites have been found to be present within the ACL.8, 22, 29 The increase of estrogen and relaxin hormones have been shown coincide with a subsequent decrease in the rate of collagen synthesis.11, 28 However, there is no conclusive evidence linking an increase in ACL injury to a predictable time within the menstrual cycle. Environmental conditions including weather, playing surface, footwear, and prophylactic and functional knee bracing have been examined. No conclusive studies demonstrate the effectiveness of functional knee braces in preventing non-contact ACL injuries. In 1974, Torg et al.31 developed a quantitative measurement entitled “release coefficient” to describe the force-to-weight ratio of shoe surface interaction. This work was enforced by Heidt et al.,13 who found that 73% of the 15 different types of athletic shoes tested demonstrated an “unsafe” or “probably unsafe” rating. When considering shoe design, it is important not to forgo safety for the sake of enhanced performance. Ekstrand et al.6 noted that there is an optimal range to be incorporated in shoe design – one that will minimize rotational friction in order to avoid injury yet optimize transitional friction to allow peak performance when performing activities such as cutting and decelerating.

In consideration of the risk factors discussed above, a group of physicians, biomechanists, physical therapists, and athletic trainers met in Hunt Valley, Maryland in June 1999 to address this ambiguity. The goal of this conference was geared toward developing a strategy to prevent ACL injuries.10 The group did a thorough review of the existing literature and conferred upon several things. In regard to environmental, anatomic, and hormonal risk factors, there is no conclusive evidence that would indicate any one single risk factor directly correlating with an increase in severe anterior cruciate ligament injuries in the female athlete population. Therefore, the emphasis has turned to biomechanical risk factors and the utilization of neuromuscular and proprioceptive intervention programs to address potential biomechanical deficits.

Prevention programs focusing on skiing, basketball, and soccer have been performed in the past with results ranging in an overall reduction of severe ACL injuries from 72 to 89%.3, 7, 14, 15, 16, 23

Henning implemented a prevention study in two Division I basketball programs over a course of eight years geared at changing player technique – stressing knee flexion upon landing, using accelerated rounded turns, and deceleration with a multi-step stop. He noted an 89% reduction in the rate of occurrence of ACL injuries in his intervention group.15

Caraffa et al.3 implemented a proprioceptive balance training program utilizing 600 semiprofessional and amateur soccer players in Italy. The study consisted of a 20-minute training program divided into 5 phases of increasing difficulty. The prospective study was completed over the duration of three complete soccer seasons. Caraffa found an incidence rate of 1.15 ACL injuries per team per year in the control group compared to a .15 incidence rate in the trained athletes. These ratios demonstrate an overall 87% decrease in ACL injuries compared to the control group.

Hewitt et al.16 did a prospective analysis of 1263 male and female athletes of various sports using a neuromuscular training program. They used a 6-week intervention program consisting of stretching, plyometrics, and weight training with emphasis on proper alignment and technique. The group noted that the incidence of serious knee injury was 2.4 to 3.6 times higher in the untrained group compared to the trained group. When examining the rate of non-contact ACL injuries, five untrained female athletes sustained ACL injuries (relative injury incidence 0.26), no trained females sustained an ACL injury (0), and one male athlete sustained an ACL injury (0.05).

Ettlinger et al.7 implemented the “guided discovery” technique in Vermont that focused on avoiding high-risk behavior and positioning (i.e. “phantom foot”), recognizing potentially dangerous skiing situations, and responding quickly to unfavorable conditions. During the 1993-1994 ski season, 4,700 ski instructors and patrollers completed the comprehensive training program at 20 ski areas throughout the United States. As a result, the rate of serious knee injuries decreased by 62% amongst the trained individuals compared to those who did not participate in the training program.

Heidt et al.14 studied 300 female soccer players between the ages of 14 to 18 years of age over a 1-year period. Forty-two athletes participated in the Frappier Acceleration Training Program, a 7-week preseason training program consisting of strength training, flexibility, sports specific cardiovascular exercise, plyometrics, and sports cord drills. He found that the trained group incurred a lower percentage of ACL injuries (2.4%) compared to the age matched control group (3.1%).

Conclusions & Future Directions

The results of this study indicate that a neuromuscular training program, such as PEP, may significantly reduce the incidence of severe ACL injuries in the female athlete. The results of this study are akin to those prevention programs developed for basketball, amateur and professional male soccer players, skiers and volleyball athletes respectively.3, 7, 14, 15, 16, 23 It is our position that a prophylactic training program that focuses on developing neuromuscular control of the lower extremity though strengthening exercises, plyometrics, and sports specific agilities may address the proprioceptive and biomechanical deficits that are demonstrated in the high-risk female athletic population.

It has been shown that the incidence of ACL injuries can be reduced through comprehensive neuromuscular training methods. Now, the impetus to determine the precise biomechanical changes that are being instituted from these prophylactic programs that enable these athletes to avoid severe ACL injury is crucial. It is known that a large number of non-contact ACL injuries occur during the deceleration phase of a cutting maneuver, when a rotation torque in concert with a varus/valgus moment is applied to a knee that is flexed 10 to 30 degrees.5, 19 Markolf et al.25 noted that both a varus moment and internal rotation moment at the knee will place the ACL at a greater risk for injury as opposed to valgus and external rotation moments. Sigward et al.30 has proposed that the female athletes demonstrating a combination of internal rotation and varus moments in the first 20% of the cut cycle may be at greater risk for ACL rupture. This “Pathokinetic Chain” is described as a combination of an increased hip adduction moment, a decrease in hip abduction control and increased hip adduction angles, thus placing the lower extremity in a valgus position. When this is combined with an increase in internal rotation moment and motion at the knee joint, the ACL incurs an increase in tension (N). When the ground reaction force is considered, which falls medial to the knee joint during a cutting maneuver, this may tax an already tensioned ACL ligament and lead to failure. This research group is seeking to further study the biomechanical implications of instituting the PEP Program via EMG and force plate analysis, to identify the mechanism of ACL injury, and finally, determine a precise neuromuscular intervention program that will specifically counteract the unopposed forces around the trunk and lower extremity to further decrease the incidence of severe ligamentous injury.

The relationship of gender, age, and training on the incidence of ACL injury are pivotal in developing a comprehensive neuromuscular and proprioceptive training program to decrease ACL injuries occurring in female athletes. Based on the two-year results, ACL incidence has remained consistently lower in the intervention group versus the control. A prophylactic neuromuscular and proprioceptive training program may have a direct benefit in decreasing the number of ACL injuries incurred by female athletes. This research foundation endorses further epidemiological and biomechanical studies to determine the exact mechanism of ACL injury and the most effective intervention that will effectively decrease ACL injuries in this high-risk population.

Acknowledgements

This study was made possible by a grant from the LA84 Foundation. We would like to extend a special thank you to Mick Ballesteros, PhD, Centers for Disease Control, Randall Dick, MS, NCAA, Julie Gilchrist, MD, Centers for Disease Control, Letha Griffin, MD, Steve Sampson, MA, Wayne Wilson, LA84 Foundation, and St. John’s Hospital, Santa Monica, CA.